LTE-A or LTE Advanced is currently being standardized by the 3GPP as an enhancement of LTE. LTE mobile communication systems are being deployed from 2010 onwards as a natural evolution of GSM (registered mark) and UMTS.
The section below briefly discusses the network architecture of an LTE wireless communications network. Further details may be found at www.3gpp.org.
The base station—in E-UTRAN—for LTE consists of a single node, generally termed the eNodeB (eNB) that interfaces with a given mobile phone (typically termed user equipment, or user terminal). For convenience, the term UE—user equipment—will be used hereafter.
The eNB is the radio access part of the UMTS/LTE system. Each eNB contains a radio transmitter, radio receiver, a control section and a power supply. eNB functions include radio resource management—RRM, radio bearer control, radio admission control—access control, connection mobility management, resource scheduling between UEs and eNB radios, header compression, link encryption of the user data stream, packet routing of user data towards its destination (usually to the EPC or other eNBs), scheduling and transmitting paging messages (incoming calls and connection requests), broadcast information coordination (system information), and measurement reporting (to assist in handover decisions).
Each eNB is composed of an antenna system (typically a radio tower), building, and base station radio equipment. Base station radio equipment consists of RF equipment (transceivers and antenna interface equipment), controllers, and power supplies.
The eNB hosts the physical layer (PHY), Medium Access Control layer (MAC), Radio Link Control (RLC) layer, and Packet Data Control Protocol (PDCP) layer that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. The evolved RAN performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated up-link QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of down-link/up-link user plane packet headers.
Each eNB is associated with an area of coverage, usually termed a cell. When a UE moves between cells, the radio link between the UE and the network is passed between eNBs. This procedure is termed a handover.
Initially, LTE networks comprised a plurality of eNBs that provided network coverage. The cells associated with the eNBs in this network that have large cell radius (often a few kilometers, and typically >0.5 km), are referred to as macro-cells. The cell in which the UE is currently attached is typically referred to as the serving cell.
Recently, the concept of heterogeneous networks (HetNet) has arisen. This type of network provides enhanced network performance by acknowledging that there is usually an unequal demand in various parts of the network. For example, office blocks, trains stations, and the like typically require much higher usage than other areas.
Thus, in heterogeneous networks, the current structure of eNBs will be complemented with a plurality of lower-power pico or femto eNBs that are deployed in areas of high demand. Such deployments should achieve significantly improved overall capacity and cell-edge performance in the network. These lower-power cells are often generically termed small cells.
In LTE networks, measurements for serving and neighbouring cells are performed according to how the network configures the UEs to perform the measurements. This may be based on well-defined events, for example when the serving cell signal level falls below a threshold. Serving cell measurements can be performed anytime. Whereas to measure cells on other frequencies, UE may require measurement gaps (gaps when UE is not using that receiver for listening to any other signals from other cells).
Measurement reports are sent from the UE to the network. The network specifies when the reports are to be sent. This may be periodical, or based on well-defined events (such as those defined in 3GPP TS36.331 Sec 5.5.4). It may also be a combination of event-based and periodic. Typically they are generated with given periodicities. These periodicities are 200 ms and 480 ms for serving cells and inter-frequency neighbouring cells.
LTE networks use a criteria called an s-measure. The s-measure is a value indicating to the UE when it should start measurement for neighbouring cells in preparation for a handover. If the UE determines that a cell's RSRP drops below a certain value (after L3 filtering), the UE will perform appropriate measurements of neighbouring cells on the frequencies and RATs indicated in the relevant measObject.
When the s-measure criteria is configured, the UE initiates neighbour cell measurements when said s-measure criteria is satisfied (that might eventually lead to the eNB triggering Handover procedure). The s-measure criteria is not suitable for detecting the presence of a small cell, as measurements, and hence handover to candidate small cell(s) will not be triggered if the UE is close enough to a macro eNB, and off-loading will not be performed.
The present invention has been devised with the above problems in mind, and in particular to propose changes to the criteria for triggering measurements to allow more straightforward off-loading of UEs to small cells.
Sections of the specification refer directly to the LTE technical specification. Full details may be found at www.3gpp.org.